One of the risk factors to knee injury and arthritis is a congenital or genetic malalignment of the lower extremity. We commonly refer to legs that are either bow-legged (varus) or knock-kneed (valgus) as being malaligned. When the leg is not perfectly straight, the weight-bearing access of the leg is not balanced and this can lead to ligament, cartilage, and meniscus problems in some patients. When planning treatment for injury to the knee one important consideration is the axial alignment of the leg.
The truth is, many people have “malalignment” of the lower extremity. For some people this does not lead to any significant problem. However, it is also common for people with varus or valgus knees to develop significant damage over time. There are many factors which contribute to knee injuries and damage, including genetic predisposition, trauma, and activity. All of these factors interact to affect the risk and degree of severity of knee injuries and problems.
Figure 1: Diagram showing a straight (balanced) leg on the right and a varus or “bow-legged” leg on the left. Note that the weight bearing axis, seen as a dotted line, goes through the inside part of the varus knee — leading to a weight imbalance (represented by the orange lines).
When a person walks, weight is transferred from leg to leg. The weight of the body is directly applied to the hip joint. At the ground level, the body's weight is applied through the center of the ankle to the ground surface. The weight of the body is transferred from the hip to the ankle in a plumb line represented by the dotted black line in figures 1 and 2. Where this broken black line passes through the knee joint determines how weight is distributed across that joint.
Figure 2: Diagram showing a straight (balanced) knee on the left and a valgus or “knock-kneed” leg on the right. Note that the weight bearing axis, seen as a dotted line, goes through the outside part of the valgus knee — leading to a weight imbalance (represented by the orange lines).
In the "balanced" knee, weight is transferred directly through the middle of the knee joint. This causes weight to be divided across both of the weight bearing joints. This situation is analogous to a well-balanced tire. If the weight is evenly distributed, it is uncommon for either the medial or lateral joint of the knee to become overburdened and sustain damage.
In the "varus" knee, the weight-bearing plumb line passes largely through the medial joint of the knee. This results in the majority of the bodyweight being passed through that joint. Similar to a tire that is poorly balanced, wear is uneven in this situation and the medial joint is more likely to sustain damage. This situation is dramatically worsened by obesity or overuse.
In the second diagram, a “normal” knee and a valgus knee are compared. The valgus knee also results in a similar uneven distribution of weight, this time overloading the lateral or outside joint of the knee. Again, it is important to recognize that weight and overuse can compound the effects of this imbalance.
The imbalance of weight associated with malalignment can cause a variety of different problems. In some knees, an overloaded compartment will result in a meniscus tear. It is also possible to have significant articular cartilage damage from overload and even possible for ligament injury or re-injury after repair to occur as a result of malalignment.
This is a relatively simple explanation of lower extremity alignment. The reality is that there are many different complex variations of alignment and for any individual patient the effects of alignment should be carefully considered by an orthopedist trained in alignment issues. Alignment is not the only cause of knee injury is only one of a variety of issues to consider when assessing a knee problem.
In some patients, it is very difficult to determine or quantify alignment without a long leg weight bearing x-ray. This x-ray can be difficult to obtain, as very few centers have the technology and equipment necessary. Most regional knee experts have established a local location where such x-rays can be made available.